Flash - EEPROM memory array and biasing method thereof

Abstract: A flash-EEPROM memory array (35) presenting a NOR architecture wherein the memory cells (36), organized in rows and columns and having drain regions connected to respective bit lines (BL), source regions connected to a common source line (BLS), and control gate regions connected to respective word lines (WL), present an asymmetrical structure wherein one of the source and drain regions presents a highly resistive portion to permit programming and erasing of the cells at different regions. The array (35) includes bias transistors (41) arranged in a row (40) and each connected between a respective bit line and the common source line (BLS), for maintaining at the same potential the drain and source regions of the cells connected to the nonaddressed bit lines during programming, and so preventing spurious writing.

Description: The present invention relates to a flash-EEPROM memory array biasing method. Nonvolatile memories have progressed rapidly from read-only to electrically erasable programmable read-only memories or EEPROMs. Of these, great interest is currently being shown in the flash type, the cells of which are based on a stacked gate structure, and are programmed (written) by channel hot electron injection at the drain region, and erased by Fowler-Nordheim tunneling. Flash memories, like other nonvolatile memories, may be variously organized according to a row and column cell arrangement wherein the cells in the same row are connected at the control gate terminal to the same word line, and the cells in the same column are connected at the drain terminal to the same bit line. In one known configuration, known as NOR architecture, the source terminals of at least part of the array are connected to a common source line, so that each bit line sees the cells connected to it in a parallel configuration (NOR connection). At present, flash memories are read/programmed byte by byte or word by word, and are erased globally (full chip erase) or on a particular portion of the overall array (sector erase). Also, most present generation flash memory devices employ a dual-voltage power supply, typically V CC =5V and V PP =12V, despite the availability of several single power supply solutions. Standard flash memories of the above types cannot operate with a supply voltage V CC of less than 5V due to the high programming current required. To reduce the programming current, new cell structures have been proposed which, as opposed to drain side injection as in standard cells, provide for source side injection hot electron programming. These new cells have aroused considerable interest by permitting programming of the cells with a lower current as compared with standard cells, and by enhancing programming efficiency by one order of magnitude. One example of an SSI programming structure is described in US Patent US-A-4,794,565 granted on 27.12.88, and comprises asymmetrical drain and source regions. More specifically, a highly resistive region is provided close to the source region, so that its conductivity is not easily modulated by the control gate voltage; and programming is based on bringing the high lateral electric field in the silicon from the drain to the source where the vertical field in the oxide is highest. Ideally, there should be no current flow between the drain and source regions during programming. In actual practice, however, the current cannot be entirely eliminated, but is considerably lower with respect to the standard structure. Other structures have also been proposed to improve the above basic concept. One such cell is described, for example, in EP-A-O 530 644 filed on 25.8.92 (priority 30.8.91) by Texas Instruments, and in an article entitled "Buried Source-Side Injection (BSSI) for Flash EPROM Programming" by Cetin Kaya, David K.Y. Liu, Jim Paterson, Pradeep Shah, IEEE Electron Device Letters, Vol. 13, No. 9, September 1992, and is based on the presence of a low-doped region implanted deeply in the channel region, adjacent to the source region, and therefore beneath the floating gate region but separated from the surface of the substrate. BSSI or SSI cells are programmed by grounding the source terminal, and applying a high voltage (10-13 V) to the gate terminal and a lower voltage (3-3.5 V) to the drain terminal, so that the applied electric field produces electron flow from the low-doped region to the surface of the substrate. The electrons, accelerated by the high field value, travel through the gate oxide and are trapped in the floating gate region, thus writing the cell. SSI or BSSI cells are erased by applying a high negative voltage (-10 V) to the control gate terminal and a low voltage (4-6 V) to the drain terminal (with a floating source terminal) to produce electron flow from the floating gate to the drain region. As such, programming and erasing are performed at different regions (source for programming, drain for erasing). SSI or BSSI cells are read by inverting the drain and source terminal bias with respect to standard cells, to achieve a sufficient read current, and by applying a high voltage (supply voltage V CC of, say, 5 V) to the gate terminal, a low voltage (1-2 V) to the source terminal, and grounding the drain terminal. It is an object of the present invention to provide a biasing method for a NOR flash type memory array which provides for low current and high programming efficiency without modifying auxiliary elements such as decoding, sensing and load devices. According to the present invention, there is provided a flash-EEPROM memory array biasing method as claimed in Claim 1. A preferred non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a known deep-implanted asymmetrical cell; Figure 2 shows the architecture of a memory array with a standard NOR configuration; Figure 3 shows the architecture of the memory array to which the biasing method according to the present invention is applied. Number 1 in Figure 1 indicates a deep-implanted asymmetrical cell as described in the aforementioned Patent Application EP-A-O 530 644, and comprising a substrate 10 housing highly doped regions 11 and 12 facing surface 13 of the substrate. According to the above European patent application, regions 11 and 12 are respectively source and drain regions separated by a channel 14; source region 11 comprises a low-doped portion 15 (high resistivity region); over channel 14, there is provided a floating gate region 16 fully embedded in a dielectric layer 17; over region 16, there is provided a control gate region 18 connected to a word line (not shown) of the memory -array of which cell 1 forms part; and floating gate region 16 is superimposed over part of drain region 12, over low-doped portion 15, and over none of the remaining (highly doped) portion of source region 11. As indicated above, BSSI cell 1 is so biased as to generate a high voltage between the tip of the low-doped region 15 and surface 13 of the substrate, and produce in the same direction a flow of electrons which are energized by the high field. Figure 2 shows a memory array 25 with a standard NOR configuration, and comprising a number of cells 26 arranged in rows and columns and divided into sectors, each sector comprising a predetermined number of local or global decoding columns, e.g. 256. The cells in the same row in each sector present the gate regions connected to the same word line WL0-WL3,..., and the cells in the same column in each sector present the drain regions connected to the same bit line BL0-BL3,..., BL16, BL17,... . In a manner not shown, the bit lines of some or all the sectors may be formed by one continuous line or divided into a number of parts. The source regions of cells 26 in each row of the sector are mutually connected by a source line 28 common to each two adjacent rows. For each sector, a number of common source lines SL are provided extending parallel to the bit lines, e.g. one every 16 bit lines, and contacting source lines 28, so that all the cells in the same sector present the source regions connected to one another. Common source lines SL define subsectors 29, in this case comprising sixteen columns (bit lines), and are grounded or connected to supply by a pair of switches (not shown) forming part of the external circuitry of the memory array and controlled alternately in different operating modes of the memory. Figure 3 shows a NOR memory array 35 to which a biasing method in accordance with the present invention is applied; memory array 35 again comprising a number of cells 36 arranged in rows and columns and organized in sectors, so that cells 36 in the same column in each sector present the drain terminal connected to the same bit line BL0-BL3, ..., BL16, BL17;..., and cells 36 in the same row in each sector present the gate terminal connected to the same word line WL0-WL3, .... The source terminals of cells 36 in the same row are connected to the same source line 38 common to each two rows in the array; and source lines 38 are connected to common source lines BLS extending parallel to the bit lines, e.g. one every sixteen bit lines. Cells 36 are asymmetrical, the source or drain region of which presents an implanted, preferably deep-implanted, portion for exploiting the two-sided cell programming and erasing principle. Cells 36 may present any known structure of the type indicated, e.g. the one shown in Figure 1. Array 35 presents one or more bias rows for appropriately biasing the bit- lines in different operating modes (program, erase, read). More specifically, Figure 3 shows a bias row 40 formed by a number of MOS transistors 41 with the gate terminals connected to a bias word line WLB, the drain terminals connected to the respective bit line BL0-BL3, ..., BL16, BL17, ..., and the source terminals connected to a respective source line 42 identical to source lines 38. Transistors 41 are of the same type as the memory cells (e.g. N-channel), are designed for high conductivity, and may also be asymmetrical with different drain and source regions. A description will now be given of the manner in which array 35 is read, programmed and erased according to the present biasing method. In the example described, the low-doped deep region is formed at the drain region (i.e. the region connected to the bit line). READ MODE Reading is performed by applying a positive voltage (≈2 V) to the bit line to which the cell to be read is connected (addressed bit line), applying supply voltage V CC to the word line to which the cell to be read is connected (addressed word line), and grounding the common source line BLS. The bias word line WLB may be biased to the supply voltage or grounded. The other (nonaddressed) word lines are grounded, and the other (nonaddressed) bit lines are left floating. The addressed cell is thus read in conventional manner as in present generation memories. Biasing of the cells connected to nonaddressed bit lines, on the other hand, depends on the bias of bias word line WLB. More specifically, if WLB is grounded, bias transistors 41 are off and the nonaddressed bit lines are floating, so that the bit lines (if formerly biased for example to the read voltage) are gradually grounded automatically due to losses. Conversely, if line WLB is connected to V CC , bias transistors 41 are on and connect the nonaddressed bit lines to common source lines BLS, so that the nonaddressed bit lines are grounded rapidly and safely. This is useful in certain situations, for example, when noise is a problem and definite voltage levels are required, or for ensuring a reset situation. In the operating condition, each of bias transistors 41 must conduct a current of no more than roughly 1 µA, which may be achieved by working on the bias of bias word line WLB and/or on the characteristics of the transistors, more specifically by appropriately selecting their V T and g m values. PROGRAMMING MODE Programming is performed by inverting the addressed cell bias with respect to read mode, and applying the different bias voltages according to a predetermined sequence. More specifically, the word line connected to the cell to be written (programming) is brought to high voltage (10-13 V); the bit line connected to the cell to be written is grounded; and the common source line BLS is brought to an intermediate voltage (4-6 V). As such, the addressed cell may be written via the drain region with low drain-source current and high efficiency. The low biasing current (few µA) required by the above biasing method poses no problems as regards formation and sizing of the program load line, so that the current driven by bias transistors 41 is in the order of tens of µA, which value is achieved automatically by virtue of the geometry of transistors 41, if asymmetrical like cells 36, or else by appropriate design choices. Such biasing, however, may possibly result in spurious writing of virgin (nonwritten) cells connected to the addressed word line and having the gate terminal biased to the high write voltage, and the source terminal connected to the same intermediate voltage as on BLS. In fact, if the bit line of even one of said cells is pre-discharged to 0 V, the bit line, being equivalent to a distributed (albeit small) capacitance, tends to charge to the voltage at the source terminal of the nonaddressed cell. This charge produces a small displacement current which, in view of the high write efficiency of the memory array cells, results in slight writing of the nonaddressed cell with a threshold jump of a few mV. As this is repeated in successive programming cycles, the nonaddressed cell charges to higher and higher threshold values until it eventually reaches the typical threshold of a written cell, thus altering the stored data. To prevent spurious programming of the cells, according to the biasing method of the present invention, the nonaddressed cells on the addressed row (and, in general, all the cells connected to the nonaddressed bit lines) are so biased as to present a zero potential difference between the drain and source terminals, thus preventing them from being written. Such biasing is achieved by means of bias transistors 41 which connect the nonaddressed bit lines to common source line BLS. More specifically, for safely preventing spurious writing even only for a fraction of programming time, the intermediate voltage is first applied to common source line BLS, and bias transistors 41 are turned on by applying a high voltage to bias word line WLB, so that all the bit lines BL are brought to the same potential as source lines 38, and all the cells present a zero voltage drop between the source and drain. Subsequently, the addressed word line is brought to the high write voltage (10 V), and the addressed bit line is grounded (0 V) so that only the addressed cell is written. ERASE MODE Erasing of a whole sector or the whole array is effected by applying a high negative voltage to the word lines and bringing the common -source line BLS to a positive voltage derived directly from supply V CC . The drain terminal is left floating. Erasing may be performed using any standard erase architecture or algorithm (e.g. feedback resistors at the source terminal to limit band-to-band tunnel current). The table below shows a summary of the above bias voltages. MODE BL a BL i WL a WL i WLB BLS READ 2V 0V/Fl V CC 0V V CC /0V 0V PROGRAM 0V 6V 10V 0V 10V 6V ERASE Fl Fl -10V -10V 0V 6V In Table I, BL a , WL a indicate the addressed bit and word lines; BL i , WL i the nonaddressed bit and word lines; and WLB, BLS the bias word lines and common source lines shown in Figure 3. According to a variation of the present invention, the deep-implanted region (region 15 in Figure 1) constitutes the source region, i.e. the region, in memory array 35, connected to the respective source line; and the region without the deep-implanted region (12 in Figure 1) constitutes the drain region, i.e. the region connected to the respective bit line. As such, the erase terminal (at intermediate voltage) is the drain terminal connected to the bit lines, and the programming terminal is the source terminal. In this case also, when programming an addressed cell, bias transistors 41 prevent the cells in the same row from being written by maintaining all the nonaddressed bit lines at the same potential as the source lines; and the biasing sequence again comprises biasing the common source line BLS to 0 V and the bias word line WLB to high voltage, biasing the addressed word line to high voltage, and, finally, biasing the addressed bit line to intermediate voltage. Table II below shows biasing of the cells according to the above variation. MODE BL a BL i WL a WL i WLB BLS READ 0V 2V/Fl V CC 0V V CC /0V 2V PROGRAM 6V 0V 10V 0V 10V 0V ERASE 6V 6V -10V -10V 0V Fl BL a , BL i , WL a , WL i , WLB and BLS in the above table have the same meaning as in Table I. In the above emdobiment, the program load line is connected conventionally to the column decoding circuitry, whereas reading requires a column load arrangement for drawing current towards ground instead of toward supply. The above biasing method and architecture thus provide for two-sided cell programming and erasure by virtue of a highly resistive region, using a NOR configuration with no alterations to the decoding, load or sensing structures, and so exploiting the vast experience acquired of standard type flash memories with a high degree of reliability and straightforward design. More specifically, the slight increase in the area of the array due to the bias transistors (which, in the case of sectors comprising 64 to 256 word lines, is nevertheless in the order of 1-2%) and in complexity due to the biasing sequence in program mode are more than compensated for by the high degree of reliability afforded by reducing the stressed cells. By virtue of enabling the cells to be programmed with a lower current as compared with standard arrays, the biasing method according to the present invention is especially advantageous in the case of even low-voltage (3 V) single supply memories. Clearly, changes may be made to the method as described and illustrated herein without, however, departing from the scope of the present invention. In particular, it applies to memory arrays with both drain terminal and source terminal programming. Moreover, provision may be made for one common source line BLS for the whole array, or for a number of source lines, one for each sector or subsector; and the subsectors may be defined in various ways, e.g. like the array portion comprising sixteen bit lines, defined by two common source lines.

Claims: A method of biasing a flash-EEPROM memory array (35) comprising at least one number of memory cells (36) organized in rows and columns and having drain regions connected to respective bit lines (BL), source regions connected to a common source line (BLS), control gate regions connected to respective word lines (WL), and floating gate regions (16); the array further comprising a number of bias switch means (41), one for each column of said array (35); said bias switch means (41) presenting a first, a second and a control terminal; said first terminal being connected to a respective bit line (BL); said second terminal being connected to said common source line (BLS); and said control terminal being supplied with a control voltage; said method comprising the steps of programming an addressed cell of said number of cells, and erasing at least one of said number of cells; characterized in that said step of programming an addressed cell comprises the step of generating a write current between said floating gate region and a highly resistive region (15) located at one (11) of said gate and source regions by biasing said common source line (BLS) and nonaddressed bit lines (BL) to a first predetermined voltage value, thereafter biasing an addressed word line (WL) to a high bias voltage, and then biasing an addressed bit line (BL) to a second predetermined voltage value; and said erasing step comprises the step of generating an erase current between said floating gate region and the other (12) of said source and drain regions. A method as claimed in Claim 1, characterized in that said step of programming an addressed cell comprises the step of generating said write current between said drain region and said floating gate region of said addressed cell; and said step of erasing at least one cell comprises the step of generating an erase current between said floating gate region and said source region of said addressed cell. A method as claimed in Claim 1, characterized in that said step of programming an addressed cell comprises the step of generating said write current between said source region and said floating gate region of said at least one cell; and said step of erasing at least one cell comprises the step of generating an erase current between said floating gate region and said drain region of said at least one cell. A method as claimed in one of Claims 1-3, comprising the step of reading an addressed cell; characterized in that said read step comprises the step of connecting nonaddressed bit lines to said common source line.

Inventors: DALLABORA MARCO, SALI MAURO LUIGI, TASSAN CASER FABIO, VILLA CORRADO, DALLABORA, MARCO, SALI, MAURO LUIGI, TASSAN, CASER FABIO, VILLA, CORRADO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L29
IPC codes: H01L29, G11C16, H01L21, H01L27